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FreeBSD-SA-13:06.mmap                                       Security Advisory
                                                          The FreeBSD Project

Topic:          Privilege escalation via mmap

Category:       core
Module:         kernel
Announced:      2013-06-18
Credits:        Konstantin Belousov
                Alan Cox
Affects:        FreeBSD 9.0 and later
Corrected:      2013-06-18 07:04:19 UTC (stable/9, 9.1-STABLE)
                2013-06-18 07:05:51 UTC (releng/9.1, 9.1-RELEASE-p4)
CVE Name:       CVE-2013-2171

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.

0.   Revision History

v1.0  2013-06-18 Initial release.
v1.1  2013-06-21 Corrected correction date.
                 Added workaround information.

I.   Background

The FreeBSD virtual memory system allows files to be memory-mapped.
All or parts of a file can be made available to a process via its
address space.  The process can then access the file using memory
operations rather than filesystem I/O calls.

The ptrace(2) system call provides tracing and debugging facilities by
allowing one process (the tracing process) to watch and control
another (the traced process).

II.  Problem Description

Due to insufficient permission checks in the virtual memory system, a
tracing process (such as a debugger) may be able to modify portions of
the traced process's address space to which the traced process itself
does not have write access.

III. Impact

This error can be exploited to allow unauthorized modification of an
arbitrary file to which the attacker has read access, but not write
access.  Depending on the file and the nature of the modifications,
this can result in privilege escalation.

To exploit this vulnerability, an attacker must be able to run
arbitrary code with user privileges on the target system.

IV.  Workaround

Systems that do not allow unprivileged users to use the ptrace(2)
system call are not vulnerable, this can be accomplished by setting
the sysctl variable security.bsd.unprivileged_proc_debug to zero.
Please note that this will also prevent debugging tools, for instance
gdb, truss, procstat, as well as some built-in debugging facilities in
certain scripting language like PHP, etc., from working for unprivileged
users.

The following command will set the sysctl accordingly and works until the
next reboot of the system:

    sysctl security.bsd.unprivileged_proc_debug=0

To make this change persistent across reboot, the system administrator
should also add the setting into /etc/sysctl.conf:

    echo 'security.bsd.unprivileged_proc_debug=0' >> /etc/sysctl.conf

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch http://security.FreeBSD.org/patches/SA-13:06/mmap.patch
# fetch http://security.FreeBSD.org/patches/SA-13:06/mmap.patch.asc
# gpg --verify mmap.patch.asc

b) Apply the patch.

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:http://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/9/                                                         r251902
releng/9.1/                                                       r251903
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing XXXXXX with the revision number, on a
machine with Subversion installed:

# svn diff -cXXXXXX --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing XXXXXX with the revision number:

<URL:http://svnweb.freebsd.org/base?view=revision&revision=XXXXXX>

VII. References

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-2171>

The latest revision of this advisory is available at
<URL:http://security.FreeBSD.org/advisories/FreeBSD-SA-13:06.mmap.asc>
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